The present invention relates to image sensors and, more specifically, to flat panel image sensors employing photosensitive thin film transistors.
Flat panel image sensors are commercially successful products that are able to effectively detect electromagnetic radiation in and near the visible light spectrum. Flat panel image sensors are fabricated by depositing and patterning various metals, insulators, and semiconductors on glass substrates as is done in flat panel displays. Such sensors commonly employ photosensitive elements, such as amorphous silicon (a-Si) PIN diodes. The photosensitive element is coupled to a readout switch, such as thin film transistor (TFT), that provides data indicative of received light.
A common use for flat panel image sensors is for medical and industrial applications to detect X-rays. The image sensor includes a phosphorescent screen that overlays an array of image sensing elements. The phosphorescent screen converts received X-rays to visible light. The array receives the visible light and generates a photocurrent responsive to the light. The photocurrent is read out as data indicative of the sensed light.
The arrays are difficult to manufacture since separate process steps are required to construct the PIN diodes and the TFTs. The total mask count may be 8 or more which is burdensome while the yields are low. Furthermore, a-Si PIN diodes are not a standard device in flat panel display processing which increases manufacturing expense. It would therefore be advantageous to use a standard process to greatly reduce the cost of flat panel image sensors.
Manufacturing TFTs for flat panel applications is a common process. A common use for TFTs is in active matrix liquid crystal displays (AMLCDs). Each TFT functions as a switch for a pixel in a matrix display. The voltage across each pixel is controlled independently and at a high contrast ratio. TFTs may be fabricated by depositing and patterning metals, insulators, and semiconductors on substrates through methods well known in the art. TFTs typically employ a-Si, polycrystalline silicone, or CdSe film as the semiconductor material. A-Si is typically used in flat panel applications as it is easily deposited on large area glass substrates at temperatures below 350 centigrade.
TFTs are more economical to fabricate than a-Si PIN diodes and are well suited for flat panel applications. The present inventors have recognized that if both the image sensing element and the readout switch of an image sensor array were incorporated as TFTs, fewer photomasks would be required and manufacturing costs would be greatly reduced.
TFTs have not commonly been used as photosensitive elements. United States Patent Application Publication Nos. 2001/0055008 and 2001/0052597, both to Young et al. (hereinafter the xe2x80x9cYoung applicationsxe2x80x9d), disclose the use of TFTs as light sensing elements for a display device. The light sensing elements provide feedback to progressively adjust the current flow through display elements to control light output. However, the use of TFTs exclusively for an image sensor is not disclosed. Since a TFT is more economical to manufacture and has already been successfully incorporated into flat panel applications, the present inventors have recognized that it would be advantageous to employ TFTs in image sensors.
An image sensor array includes image sensors disposed on a substrate and arranged to receive and sense an image. Each image sensor represents a pixel for a received image. The image sensors each include a photo TFT that generates a photocurrent in response to the image. The photo TFT may be manufactured using common processes for TFTs in flat panel applications. The photo TFT has a gate electrode which is shorted to its source electrode to obtain a photocurrent that is substantially independent of source-drain bias. The photo TFT may also be configured with interdigitated source and drain electrodes to increase the photosensitivity.
Each photo TFT is coupled to a bias line to enable operation and a storage capacitor to store a charge and discharge upon generation of a photocurrent. The storage capacitor is coupled to a readout TFT that is also manufactured using known processes. The readout TFT passes a charge from the storage capacitor to a data line. Operation of the readout TFT is enabled by a select line which is coupled to the gate electrode of the readout TFT. A light shield may be disposed over the channel of the readout TFT to prevent a charge leaking through the readout TFT.
Each photo TFT may further be coupled to a reference TFT. A reference TFT is similar in structure to a corresponding photo TFT and provides an equivalent dark current to compensate for dark current in the photo TFT. The reference TFT includes a light shield so as to not generate a photocurrent in response to received light.
The photo TFTs can provide an effective and economical alternative to conventional photodiodes. Photo TFTs may be manufactured with corresponding readout TFTs using conventional methods thereby reducing mask counts and costs. Photo TFTs may further yield photocurrents greater than that of photodiodes.
Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings.